The present application relates to a nonaqueous secondary battery, and particularly to a negative electrode in which silicon or the like is used for forming a negative electrode active material layer so as to realized an enhanced capacity, a battery using the negative electrode, and a method of manufacturing such a negative electrode.
In recent years, mobile apparatuses have been enhanced in performance and in the number of their functions, and secondary batteries used as power supplies for the mobile apparatuses have been demanded to be reduced in size, thickness and weight, and to be enhanced in capacity. As a secondary battery capable of meeting the demands, there is the lithium (Li) ion secondary battery.
The characteristic features of the Li ion secondary battery vary largely depending on the electrode active materials used therein, and the like. In a typical Li ion secondary battery in practical use at present, lithium cobalt oxide is used as a positive electrode active material, and graphite is used as a negative electrode active material. However, the capacity of the Li ion secondary battery with this configuration is approaching to the theoretical value and, therefore, it is difficult to remarkably enhance the battery capacity by improvements in the future.
It has been known that the use of metallic Li as a negative electrode active material makes it possible to obtain a battery showing a high energy density. Such a battery, however, has the problem that when charge and discharge are repeated, deposition and dissolution of Li are repeated at the negative electrode, and Li would be deposited in a dendrite form on the negative electrode, leading to internal short-circuit. In order to obviate this problem, therefore, it has been proposed to replace the metallic Li with another active material, examples of which include transition metal chalcogenides; transition metal oxides; tin (Sn), silicon (Si), lead (Pb) and the like elements capable of alloying with Li, and alloys of these elements with Li. Now, the related art in which a material capable of alloying with Li is used as a negative electrode active material will be described below.
Patent Document 1 (set forth later) with the title “Electrode for Battery, and Lithium Secondary Battery Having the Electrode” contains the following description.
A Li secondary battery is a secondary battery having at least a negative electrode, a separator, a positive electrode, and an electrolyte or an electrolytic solution, wherein the negative electrode includes at least a metallic element capable of alloying with Li and a metallic element incapable of alloying with Li as constituents, and an output terminal on the negative electrode side is led out from a part of the metal incapable of alloying with Li. Where a metal incapable of alloying with Li is thus disposed at a negative electrode current collecting part, it is possible to restrain the current collecting ability from being lowered by pulverization and cracking which would arise from the deposition and dissolution of Li at the times of charge and discharge.
In addition, the negative electrode may be formed by a method in which a powdery material containing a metallic element incapable of alloying with Li is bound by a binder to a current collecting member formed from a metal incapable of alloying with Li, followed by or not followed by baking. With the negative electrode formed in this manner, it is possible to restrain the fatigue failure which might arise from repetition of both expansion due to alloying with Li at the time of charging and contraction due to dissolution of Li at the time of discharging.
Patent Document 2 (set forth later) with the title “Negative Electrode Material for Nonaqueous Secondary Battery, and Method of Manufacturing the Same” contains the following description.
A negative electrode active material is a semiconductor thin film including a simple material, or its compound, of at least one element which is selected from the group consisting of Group 3B, 4B and 5B elements and which is capable of insertion and extraction of Li ions. The semiconductor thin film is formed over a copper or stainless steel substrate, which is used as a current collector, by any one selected from among vacuum thin film forming methods such as vapor deposition, sputtering, ion plating, CVD, etc. Further, the semiconductor thin film formed by the vacuum thin film forming method is heat treated in a vacuum.
Patent Document 3 (set forth later) with the title “Electrode for Lithium Battery, and Lithium Battery and Lithium Secondary Battery Which Use the Same” contains the following description.
According to a first aspect, there is provided a Li battery electrode including an active material capable of insertion and extraction of Li, wherein non-crystalline Si is used as the active material. The “non-crystalline Si” includes amorphous Si and microcrystalline Si.
According to a second aspect, there is provided a Li battery electrode having, provided over a current collector, a thin film including an active material capable of insertion and extraction of Li, wherein the thin film is split into columnar parts by cuts formed in the thickness direction thereof, and a bottom part of each of the columnar parts is in adhesion to the current collector. A gap is formed in the periphery of each columnar part, and the presence of the gap relaxes the stresses arising from the expansion and contraction of the thin film attendant on the charge-discharge cycles, whereby a stress leading to exfoliation of the active material thin film from the current collector can be restrained from being generated. Therefore, a good adhesion condition can be maintained between the bottom part of each columnar part and the current collector.
The structure in which the Si thin film is split into the columnar parts by the cuts formed therein promises an increased area of contact between the Si thin film and the electrolytic solution. In addition, it is considered that, since the columnar parts are formed in substantially equal sizes, the charge-discharge reactions attended by insertion and extraction of Li proceed efficiently in the active material thin film.
It is also considered that, since each of the columnar parts of the Si thin film is in adhesion to the current collector, the active material is electrically connected to the current collector in a good condition, so that the charge-discharge reactions can proceed efficiently. Besides, an upper part of each columnar part has a rounded shape, so that concentration of current will hardly occur; in this electrode structure, therefore, there is little possibility of a dendritic Li metal deposition reaction or the like.
It is considered that, even when the Si thin film split into the columnar parts is subjected to charge-discharge cycles, the stresses arising from the expansion and contraction of the active material are relaxed by the presence of the gaps formed in the peripheries of the columnar parts, so that the charge-discharge cycle can be repeated without bringing about exfoliation of the active material from the current collector.
According to the first and second aspect, it is possible to obtain a lithium secondary battery with a high charge/discharge capacity and exbatteryent charge-discharge cycle characteristics.
Patent Document 4 (set forth later) with the title “Negative Electrode and Battery Using the Same” contains the following description.
A negative electrode includes a negative electrode active material layer having pluralities of first layers and second layers laminated alternately, the first and second layers containing Si and differing from each other in oxygen (O) content. According to the negative electrode and a battery using the negative electrode, it is possible to restrain severe expansion and contraction of the negative electrode active material layer and to restrain structural breakdown of the negative electrode active material layer. Further, this can reduce in reactivity in negative electrode active material layer and electrolyte. Therefore, cycle characteristics can be enhanced, and practical cycle characteristics can be obtained even when the thickness of the negative electrode active material layer is brought to a practical level.
Patent Document 5 (set forth later) with the title “Method of Manufacturing Electrode for Lithium Secondary Battery” contains the description of a method of forming an active material layer over a current collector by use of an electrode forming device based on a vapor deposition process.
[Patent Document 1]
Japanese Patent Laid-open No. Hei 8-50922 (paragraphs 0011-0014 and 0017-0018; FIGS. 2 and 3)
[Patent Document 2]
Japanese Patent Laid-open No. Hei 11-135115 (paragraphs 0004-0005)
[Patent Document 3]
Japanese Patent Laid-open No. 2002-83594 (paragraphs 0006, 0008, 0020, 0022, 0185 and 0350)
[Patent Document 4]
Japanese Patent Laid-open No. 2004-349162 (paragraphs 0006-0008 and 0067; FIGS. 1 to 3)
[Patent Document 5]
Japanese Patent Laid-open No. 2005-158633 (paragraphs 0041-0044; FIGS. 4 to 5)
In recent years, vigorous investigations have been made on enhancement of the capacity of Li ion secondary batteries in which Si, Sn or the like material capable of alloying with Li is used as a negative electrode active material. For example, in the case where Si is used, an enhanced capacity is obtained, but the negative electrode would be broken due to the expansion and contraction attendant on the charge and discharge, leading to worsening of charge-discharge cycle characteristics.
Thus, where Si or Sn is used as the negative electrode active material, the expansion and contraction of the negative electrode active material layer arising from the repeated charging and discharging are so large that the negative electrode active material layer may be crushed and pulverized or may come off from the negative electrode current collector, leading to a deteriorated current collecting ability, a lowered charge/discharge capacity, a lowered load characteristic, or lowered charge-discharge cycle characteristics.
As compared with a negative electrode having a negative electrode active material layer formed over a negative electrode current collector from a negative electrode active material by a sintering method using a binder, a coating method or the like method, a negative electrode having a negative electrode active material layer formed by vapor-phase growth does not have a path for insertion/release of Li ions, so that it involves the problems that charge and discharge will not proceed uniformly, that the expansion and contraction of the negative electrode active material layer cannot be relaxed, and so on.
Patent Document 3 contains the description of an active material thin layer comprised of columnar bodies, but there is no sufficient consideration of how to make charge and discharge proceed uniformly. In order to realize an enhanced capacity and to enhance the load characteristic and charge-discharge cycle characteristics, it is important to secure uniform progress of the charge and discharge in the negative electrode active material layer, and a technology for realizing this is being demanded.